A number of known analysers are now described, particularly in relation to the field of scanning electron microscopy.
Scanning electron microscopes (SEMs) are common tools for the observation of small features. A finely focused electron beam (“probe”) with an energy typically between about 1 keV and 20 keV is scanned across the surface of the sample in question. At each probe position on the surface, signals are measured by a number of different types of detector. Commonly, scattered electrons and x-rays are detected and for each signal, magnified images of the specimen are constructed where the signal strength modulates the intensity at positions corresponding to the probe positions scanned on the surface. At each probe position, the energy spectrum of the scattered electrons and x-rays is determined by the composition and topography of the sample surface and underlying bulk material. At present compositional analysis in an SEM is most commonly achieved with x-ray detection.
When the SEM is operational, electrons comprising the incident probe penetrate the specimen and follow irregular paths due to sideways scattering. X-rays, Auger electrons and secondary electrons are produced throughout the electron trajectory envelope which can extend to some microns below the surface. The effective probe diameter therefore broadens with depth and the information volume is large. This can make the identification of thin layers and features below 1000 nm in dimension difficult. Low energy electrons have a very limited range and only escape from layers very close to the surface where the probe is still narrow. In the majority of SEMs, low energy secondary electrons are detected to form a high resolution “secondary” electron image of the sample which shows topographic contrast but gives little information about the composition of the specimen. In principle, high resolution images can also be obtained using the signal from low-energy Auger electrons and, if the detector is energy-selective, characteristic Auger emissions can be used to identify some elements present in the specimen. While Auger spectrometry is commonly available on Ultra High Vacuum (UHV) “surface analysis” microscopes (typically 10−9 torr), it is rarely available in conventional SEMs (<10−5 torr) partly because of the size and expense of high resolution electron spectrometers.
Backscattered electrons are the higher energy electrons that, after entering via the incident beam, “bounce” back out of the specimen and hit an electron detector. These electrons give atomic number (Z) information and the information volume is less than for x-rays but the region generating the signal is generally still substantially wider than the incident probe diameter. If the incident beam voltage is reduced, the sideways scattering is less but the total backscattered signal is not always proportional to Z at low incident beam voltages.
Electrons that are backscattered out of the specimen in the early stage of the electron trajectory near the surface will have energies close to the primary beam energy and are thus “low loss electrons” (LLEs). These LLEs produce a signal that arises from a shallow region that is close to the incident beam. One way to observe this signal is to tilt the sample and observe glancing reflections because this increases the proportion of LLEs that reach a conventional backscattered electron detector (see William E. Vanderlinde, “Forward Scattered Scanning Electron Microscopy for Semiconductor Metrology and Failure Analysis”, Proceedings from the 29th International Symposium for Testing and Failure Analysis, Nov. 2-6, 2003, Santa Clara, Calif. USA). However, it is not always possible to tilt the sample sufficiently and spatial resolution and depth of field is compromised in this configuration. Alternative approaches use energy filtering to detect only LLEs. With energy filtering, the sample can be flat and spatial resolution is improved compared with the tilted sample method. In addition, the signal depends on Z even down to low beam voltages (Applied Surface Science, Vol. 120, pg 129-138, 1997 I. R. Barkshire, R. H. Roberts and M. Prutton).
One method of detecting the LLEs is to use a spherical grid retarding analyser as described in U.S. Pat. No. 5,408,098. Spherical grid retarding analysers occupy a large volume around the sample, require a fixed sample/analyser position and the presence of even small stray magnetic fields degrades the energy resolution considerably.
Published US patent application US20040245465 (and extensions described in http://www.smt.zeiss.com/C1256E4600307C70/EmbedTiteIIntern/new_detection_system_for_leo_fe-sem/$File/new_detection_system_for_leo_fe-sem.pdf), shows an in-lens detector where backscattered electrons travelling back up the axis of the electron lens have to have enough energy to pass a retarding grid filter before being detected. This makes good use of the existing electron optics but the energy cut off is not sharp and energy resolution is typically limited to approximately 120 eV which limits the spatial resolution. Furthermore, in this configuration low loss electrons can only be selectively filtered for operating voltages up to 3 kV.
Rather than select all electrons up to a given loss energy, a narrow energy band can be selected using a conventional dispersion type electron energy analyser as used on surface analysis instruments.
For a concentric hemispherical analyser, a rather small solid angle of emitted electrons is accepted so the efficiency of collection is small. A transfer lens is generally used to convey electrons from the sample to the entrance slit between the hemisphere. The lens is usually electrostatic, and in some cases includes a curved mesh or grid lens to improve collection efficiency (as described in U.S. Pat. No. 4,358,680).
The cylindrical mirror analyser is an alternative although it occupies a lot of volume around the specimen region and is conventionally used in dedicated UHV surface analysis instruments where the electron gun and detector can be integrated. Here Auger electrons are directed back beyond and around the outside of the electron gun for subsequent detection. The large size can make it impractical for use in a conventional SEM fitted with multiple detectors for different types of signal.
The electron detectors described so far will record all scattered electrons in a prescribed energy range and an image is formed pixel-by-pixel by scanning an electron probe over the specimen surface. If the specimen is crystalline, then the scattered electrons will exhibit diffraction effects and diffraction patterns can be visualised at a single probe position by imaging the angular distribution of scattered electrons.
U.S. Pat. No. 6,492,644 describes a device for energy and angle-resolved spectroscopy that is suitable for imaging the angular distribution of scattered electrons in an SEM. This device uses a decelerating lens to take electrons emerging at a range of angles from the probe on the sample and form them into essentially parallel trajectories whose reciprocal distances correspond to the original angular distribution. Therefore, a flat imaging detector can be used to record a diffraction pattern and a single flat electrostatic mesh can be biased to reflect lower energy electrons so that the diffraction image is formed from higher energy electrons only. This device provides a sharp energy cut-off but the requirement to preserve the relative angular relationships of electron trajectories for diffraction analysis places restrictions on the range of source positions that can be studied. It also requires a large diameter tube to match the size of the camera and means performance is seriously degraded when there is a stray magnetic field in the vicinity of the specimen.
One object of the invention is to provide a compact analyser device that can be fitted to a wide range of conventional apparatus such as scanning electron microscopes, for detecting a large solid angular range for particles scattered out of a specimen. It is a further object that the device will analyse the energy spectrum of emitted particles with good energy resolution and can also operate when there is a magnetic field in the vicinity of the specimen. Since we are not concerned with imaging the angular distribution of scattered particles and can relax the constraints associated with this requirement it is a further object to thereby make use of non-imaging lenses and also smaller, more efficient non-imaging charged particle detectors, particularly electron detectors.